1. Field of the Invention
The present invention relates to a compound semiconductor device having so-called HEMT structure, and a method of fabricating the same.
2. Description of the Related Art
In recent years, extensive efforts have been directed to development of electronic devices using sapphire, SiC, GaN or Si as a substrate, and using a GaN layer and an AlGaN layer sequentially stacked thereon by crystal growth so as to allow the GaN layer to function as an electron transit layer. The GaN layer is largely expected for its operation under a large voltage resistance because it has a band gap energy of 3.4 eV, which is larger than 1.4 eV of GaAs.
There is a need of high voltage operation for recent amplifiers at base stations of cellular phone, where high voltage resistance is becoming essential. Zero-current voltage resistance of as large as exceeding 200 V is reported at present.
There is also a need of suppressing so-called current collapse, which is fluctuation in ON-resistance during the operation, for which it is necessary to further dispose an additional GaN layer on the AlGaN layer and to form an SiN protective film.
This configuration also makes it possible to further improve the voltage resistance if a gate electrode is formed in an over-hung geometry on the SiN protective film.
One known specific example of the compound semiconductor device having the HEMT structure is shown in FIG. 12 (see Patent Document 1).
The compound semiconductor device has, as being sequentially stacked on a substrate 101 by the epitaxial growth process, a non-doped GaN layer (i-GaN layer) 102, an AlGaN layer 103 and a GaN layer (n-GaN layer) 104 doped with an n-type impurity, and further comprises a gate electrode 106 which establishes Schottky contact with the n-GaN layer 104 through an SiN film 105 formed on the n-GaN layer 104, and a source electrode 107 and a drain electrode 108 which establish ohmic contact with the n-GaN layer 104.
The AlGaN layer 103 herein further comprises an i-AlGaN layer 111 having a thickness of approximately 3 nm and an Al composition of approximately 25%, and an n-type AlGaN layer (n-AlGaN layer) 112 having a thickness of approximately 22 nm and an Al composition of approximately 25% stacked with each other.    [Patent Document 1]    Japanese Patent Application Laid-Open No. 2002-359256
The compound semiconductor device shown in FIG. 12, however, found to have a problem in that it has a leakage current of the gate electrode larger than a specified value required for the device. That is, it is becoming clear that the compound semiconductor device has a large absolute value of the leakage current even if it is rated as 100 V or above in terms of breakdown voltage resistance or gate voltage resistance. This is because a large roughness right under the gate electrode is causative of a local concentration of electric field, and the leakage as a consequence.
The present invention was conceived after considering the above-described problems, and an object thereof is to provide a compound semiconductor device having a reduced current collapse and an improved RF characteristic so as to achieve a sufficient voltage resistance required for the amplifiers at the base stations for cellular phone, and a method of fabricating such compound semiconductor device.